This invention relates to electronic instruments for detecting and measuring RF voltage wave signals on coaxial transmission lines such as between a transmitting antenna and a transmitter. More particularly, the invention relates to an "insertion-type" RF directional wattmeter for detecting and measuring both the forward and reflected voltage wave signals on a coaxial transmission line and particularly for installation in a vehicle utilizing "Citizen' Band" (CB) radio equipment and a 50 l ohm coaxial cable between the transmitter and the antenna.
Insertion-type RF directional wattmeters are used in many applications in the RF field, particularly, in matching antennas to transmission lines and in minimizing the voltage standing wave ratio (VSWR) on the line. One application that is becoming increasingly important is in connection with CB transmitters that are currently so popular in the United States for use in automotive vehicles. These installations require accurate impedance matching in view of the limited power permitted.
Meters currently available for this application are, for example, of the type disclosed in U.S. Pat. Nos. 2,852, 741 and 2,891,221. Units embodying the inventions of these patents are high-quality type instruments of fairly expensive construction and while they are entirely suitable for CB applications they are in many instances too expensive for use by the average CB radio operator.
The principle of operation of these units is based on the use of a rigid, coaxial line section that is inserted in the coaxial transmission line such as by standard coaxial connectors. An inductive pick-up coil positioned in a transverse opening in the outer conductor is adapted for rotation about an axis normal to the axis of the line section. The pick-up coil is connected by special leads to a D'Arsonval meter movement and the resulting meter reading indicates the magnitude of the wave signal in watts, the indication being either that of the magnitude of the forward voltage wave level or the reflected voltage wave level depending upon the particular orientation of the pick-up coil.
A loop located in the electrical field between the inner and outer conductors of a coaxial transmission line has a voltage induced therein proportional to the current I in the inner conductor, there being a mutual inductance M between the loop and the transmission line and the loop being positioned in the plane of the inner conductor of the line. A series circuit of resistance R and capacitance C connected across the transmission line conductors will give a voltage across the resistance R proportional to the voltage E between the line conductors. In directional coupler and so-called reflectometers the arrangements mentioned are combined in a sampling circuit in which the resistor R is connected in series with the loop and capacitive coupling is provided as by capacitor plates or armatures on the loop and the inner conductor, or by capacitance effects between the components of the sampling circuit and the inner conductor.
Considering the sampling circuit mentioned and using lumped impedances, it is apparent that M is either positive or negative depending upon the directional relation between the loop and the wave signal energy traveling on the line.
The instrument described obtains reversal of the mutual inductance M through 180.degree. rotation of the loop relative to the transmission line. The forward traveling wave has voltage E.sub.f and current I.sub.f while the reflected traveling wave has voltage E.sub.r and current I.sub.r. Thus, if Z.sub.o be the characteristic impedance of the line and p the reflection coefficient: EQU p = E.sub.r /E.sub.f = -I.sub.r /I.sub.f
and EQU e = jw (CRE + MI) EQU = jwE.sub.f [CR(1+p) + (M/Z.sub.o) (1-p)]
where e is the total electromotive force induced in the loop or sampling circuit. The components are selected so that: EQU RC = M/Z.sub.o = K
k being a constant. If we let e be the electromotive force when M is positive so that the voltage across R and the voltage induced in the loop are additive, and let e- be the elctromotive force when M is negative and the voltages referred to are opposed, the former gives a maximum and the latter a minimum indication, thus: EQU e = jwE.sub.f [K(1+p) + K(1-p)] EQU = 2jwE.sub.f K EQU e-=jwE.sub.f [K(1+p) - K(1-p)] EQU =2jwE.sub.f K.sub.p
from which the reflection coefficient and standing wave ratio can be obtained. It is also feasible to measure power P being fed through the transmission line: ##EQU1##
As indicated above, current models available containing this system for detecting and measuring the forward and reflected RF wave signals on a coaxial transmission line are high quality, relatively expensive instruments not usually practical for CB installations. For CB installation, a model capable of operation within a limited frequency range and a carefully controlled power range are all that are required. Interchangeable pick-up cartridges are not necessary and it is not necessary that the line section be a separate portable element or that it be capable of removal from the instrument housing for operation remotely as in the case of prior art models embodying this principle.
The instrument of the present invention satisfies the requirements indicated above and affords other features and advantages heretofore not obtainable.